Acid fast organisms such as Mycobacterium have a cell wall made up of glycolipids, in particular mycolic acids and these are resistant to acid staining, only retaining the carbol fuchsin, making them appear red and gram negative.

If the organisms look purple, then these have been decolourised and therefore do not need acid fast treatment. Gram positive would be the conclusion you draw.

To make things clearer:Acid fast bacteria ARE gram positive, but the structure of their cell wall prevents the entry of the crystal violet. So they appear Gram negative. So if you have a gram positive, it is gram positive, no need for acid fast test. If it is Gram negative, it can be because it is a gram negative, or because it is an acid fast organism, hence the need for the test.

Patrick

Science has proof without any certainty. Creationists have certainty without
any proof. (Ashley Montague)